There is presently under development a position determining system, referred to as the Global Positioning System (GPS), also called NAVSTAR, wherein a multitude of orbiting space craft will be used to enable the position of certain types of receivers to be located relative to the earth. In the system that will ultimately be put into operation, there will be eight orbiting space crafts in each of three sets of orbits so there will be a total of twenty-four space crafts. The three sets of orbits will have mutually orthogonal planes relative to the diameter of the earth so that there will be two sets of polar orbits and one set of equatorial orbits. The space crafts will be in twelve hour orbits and the position of each space craft at any time will be precisely known. The longitude, latitude and altitude of any point close to earth, with respect to the center of the earth, will be calculated by determining the propogation time of electromagnetic energy from four of the space crafts to the point.
To determine the propagation time from each space craft to a point close to earth, electromagnetic energy is transmitted from each space craft to a receiver at the point. Energy on a single carrier frequency from all of the space crafts is transduced by a receiver at a point close to earth. The space crafts from which the energy originated are identified by modulating the carrier transmitted from each space craft with pseudo random type signals. In one mode, referred to as the clear/acquisition (C/A) mode, the pseudo random signal is a Gold code sequence having a chip rate of 1.023 MHz; there are 1023 chips in each Gold code sequence such that the sequence is repeated once every millisecond. (The chipping rate of a pseudo random sequence is the rate at which the individual pulses in the sequence are derived and therefore is equal to the code repetition rate divided by the number of members in the code; one pulse of the noise code is referred to as a chip.) The 1.023 MHz Gold code sequence chip rate enables the position of the receiver responsive to the signals transmitted from four of the space crafts to be determined to an accuracy of 300 meters. There is a second mode, referred to as the precise or protected (P) mode wherein pseudo random codes with chip rates of 10.23 MHz are transmitted with sequences that are extremely long, so that the sequences repeat no more than once per week, which enables the receiver position to be determined to an accuracy of approximately 10 meters. However, the P mode requires relatively complex receivers and is intended for use only by authorized receivers. Hence, civilian and/or military receivers that are apt to be obtained by unauthorized users are not responsive to the P mode.
To enable the receiver to separate the C/A signals received by it from the different space crafts, the receiver includes a plurality of different Gold code sources, each of which corresponds with the Gold code sequence transmitted from one of the space crafts in the field of view of the receiver. The locally derived and received Gold code sequences are cross correlated with each other over the one millisecond, Gold code sequence intervals. The phase of the locally derived Gold code sequence is varied, on a chip by chip basis, and then within a chip, until the maximum cross correlation function is obtained. Since the cross correlation for two Gold code sequences having a length of 1023 bits is approximately sixteen times as great as the cross correlation function of any of the other combinations of Gold code sequences, it is relatively easy to lock the locally derived Gold code sequence onto the same Gold code sequence that was transmitted by one of the space crafts. The Gold code sequences from four of the space crafts in the field of view of the receiver are separated in this manner by using a single channel that is sequentially responsive to each of the locally derived Gold code sequences, or by using parallel channels that are simultaneously responsive to the different Gold code sequences. After four locally derived Gold code sequences are locked in phase with the Gold code sequences received from four space crafts in the field of view of the receiver, the position of the receiver can be determined to an accuracy of 300 meters. The 300 meter accuracy of GPS is determined by the number of space crafts transmitting signals to which the receiver is effectively responsive, the variable amplitudes of the received signals and the magnitude of the cross correlation peaks between the received signals from the different space crafts. In response to reception of multiple PRN (pseudo range noise) signals, there is a common time interval for some of the codes to likely cause a degradation in time of arrival measurements of each received PRN due to the cross correlations between the received signals. The time of arrival measurement for each PRN is made by determining the time of a peak amplitude of the cross correlation between the received composite signal and a local Gold code sequence that is identical to one of the transmitted PRN. When random noise is superimposed on the received PRN, increasing the averaging time of the cross correlation between the received signal and a local PRN sequence decreases the average noise contribution to the time of arrival (hence distance) error. However, because the cross correlation errors between the received PRN's are periodic, increasing the averaging time increases both signal and the cross correlation value between the received PRN's alike and time of arrival errors are not reduced.
It is an object of the present invention to provide a new and improved apparatus for and method of enabling the position of a receiver responsive to a plurality of pseudo random type sequences from a plurality of sources having a known position to be determined.
It is a more specific object of the invention to provide an apparatus for and method of enabling the position of a relatively stationary receiver responsive to the C/A signals transmitted from a plurality of space crafts of the GPS to be determined to an accuracy of greater than 300 meters.
Another object of the invention is to provide an apparatus for and method of enabling the position of a receiver responsive to a multiplicity of C/A signals to be determined to an accuracy greater than 300 meters wherein a considerable portion of the apparatus utilized for determining position to an accuracy of 300 meters is employed.
While the present invention is described in connection with determining the position of a receiver responsive to the C/A signal of the GPS, it is to be understood that the principles of the invention are applicable to any system for determining the position of a relatively stationary or slowly moving receiver responsive to pseudo random type sequences. The term "slowly moving" refers to a receiver that derives an output at a particular spatial position such that a computer responsive to the output indicates the position prior to the receiver moving to another position where the computer will respond to the outputs to indicate a different position. However, if sequences and bit rates different from the C/A signal of the GPS are employed from multiple stations having known positions, the principles of the invention are applicable.